Measure max indirect prefetch distance from data prefetch.

This removes dependincy between data and metadata prefetch distance
tunables, that previously allowed data prefetch go beyond metadata,
that made no sense and also caused stream reference counting problem.

Signed-off-by: Alexander Motin <[email protected]>

Author: amotin

module/zfs/dmu_zfetch.c

@@ -392,7 +392,7 @@ dmu_zfetch_prepare(zfetch_t *zf, uint64_t blkid, uint64_t nblks,
 	 * (i.e. the amount read now + the amount of data prefetched now).
 	 */
 	pf_ahead_blks = zs->zs_ipf_blkid - blkid + nblks + pf_nblks;
-	max_blks = max_dist_blks - (ipf_start - end_of_access_blkid);
+	max_blks = max_dist_blks - (ipf_start - zs->zs_pf_blkid);
 	ipf_nblks = MIN(pf_ahead_blks, max_blks);
 	zs->zs_ipf_blkid = ipf_start + ipf_nblks;